Method of producing trimellitic acid

ABSTRACT

Disclosed is a method of producing trimellitic acid through the liquid-phase oxidation of pseudocumene in acetic acid. The oxidation comprises a) conducting a first oxidation using an initial oxidizing catalytic system at 120-200 i É for 5-20 min in an oxidizing reactor, said initial oxidizing catalytic system comprising at least three compounds selected from the group consisting of cobalt compound, manganese compound, zirconium compound and bromine compound; b) conducting a second oxidation in situ at 160-220 i É for 30-60 min under addition of an additional catalytic system, said additional catalytic system comprising at least two compounds selected from the group consisting of cobalt compound, manganese compound, zirconium compound, and bromine compound; and c) completing the oxidation of pseudocumene at a temperature from 180 to 230 i É for a time from 5 to 20 min without the addition of catalysts into the reactor. The pressure is adjusted in the range from 100 to 450 psig over the steps a), b) and c).

TECHNICAL FIELD

The present invention relates to a method of producing trimellitic acidby oxidizing pseudocumene (1,2,4-trimethylbenzene) with gas containingmolecular oxygen. More particularly, the present invention is directedto a method of producing trimellitic acid by liquid-phase oxidizingpseudocumene in an acetic acid solvent under a gas atmosphere containingmolecular oxygen using a combination of oxidizing catalytic ingredientsselected from the group consisting of cobalt, manganese, zirconium andbromine.

BACKGROUND ART

As well known to those skilled in the art, trimellitic acid is anintermediate of a synthetic resin, and dehydrated to produce trimelliticanhydride (TMA) used as a heat stable plasticizer, an epoxy resinhardening agent, or various additives for improving heat resistance.

A process of producing trimellitic anhydride using pseudocumene as astarting material comprises the steps of oxidizing pseudocumene toproduce trimellitic acid, dehydrating trimellitic acid to producetrimellitic anhydride, refining trimellitic anhydride, and distilling aused solvent to recycle the solvent. Of these steps, the oxidizing stephas the biggest effect on purity of trimellitic anhydride. The reasonfor this is that the impurities contained in the resulting trimelliticanhydride obtained from the dehydration step of trimellitic acid or thesubsequent refining step thereto, mainly consists of organic impuritiesand derivatives thereof which are by-products in the oxidation ofpseudocumene. Thus, in the case of producing trimellitic anhydride usingtrimellitic acid with low purity, it is difficult to obtain trimelliticanhydride with high purity in high yield.

Conventionally, trimellitic acid has been produced by oxidizingpseudocumene in an acetic acid solvent under a gas atmosphere containingmolecular oxygen in the presence of catalysts including one or moreheavy metal compounds and a bromine compound. For example, U.S. Pat.Nos. 4,537,978, 4,587,350, 4,755,622, and 4,764,639 are directed to amethod of producing trimellitic acid by oxidizing pseudocumene throughdiscontinuous two-step oxidation reaction. The first oxidation reactionis conducted at a relatively low temperature in the presence ofoxidizing catalysts consisting of cobalt, manganese, zirconium andbromine. The second oxidation reaction is conducted at a relatively hightemperature while adding a bromine catalyst to an oxidizing reactor,even though a temperature and compositions of pseudocumene and eachcatalyst may be slightly different from each patent. However, thesepatents are disadvantageous in that the second oxidation reaction isperformed while adding the catalyst consisting of only bromine to theoxidizing reactor, thus not obtaining a sufficiently high purity oftrimellitic acid.

Meanwhile, U.S. Pat. Nos. 4,845,275, 4,895,978, 4,992,579, and 5,250,724disclose a process of producing trimellitic acid, in which undesirableproduction of high boiling point impurities is suppressed, and agasification reaction of pseudocumene into carbon dioxide is preventedthrough complete oxidation of pseudocumene by use of catalystscomprising other metals such as lead and cerium in addition to cobalt,manganese, zirconium and bromine.

However, these patents have disadvantages in that it is not preferableto commercialize the process because the purity of trimellitic acid isnot sufficiently improved and lead and cerium are used as additionalcatalysts, which are very harmful to human body.

DISCLOSURE OF THE INVENTION

The present inventors have conducted extensive studies in order to avoiddisadvantages of the conventional techniques, resulting in the findingthat trimellitic acid is produced in high yields through three-stepoxidation reaction having different temperatures, reaction times, andcatalytic conditions. In other words, pseudocumene is firstly oxidizedin the presence of an initial oxidizing catalytic system, secondlyoxidized under the addition of an additional catalytic system to theoxidizing reactor, and then maintained for a predetermined period underthe condition of increased temperature and pressure without addition ofthe additional catalyst to produce trimellitic acid.

Accordingly, it is an object of the present invention to provide amethod for producing trimellitic acid in high purity and high yield byimproving the oxidation of pseudocumene, which has the largest effect onpurity and yield of trimellitic anhydride in a process of producingtrimellitic anhydride.

Based on the present invention, the above object can be accomplished bya provision of a method for producing trimellitic acid throughliquid-phase oxidation of pseudocumene in an acetic acid solvent under agas atmosphere containing molecular oxygen in the presence of acatalytic system containing a combination of catalytic ingredientsselected from the group consisting of cobalt, manganese, zirconium, andbromine, said oxidation of pseudocumene comprising:

-   -   a) conducting a first oxidation in the presence of an initial        oxidizing catalytic system at a temperature from 120 to 200° C.        for a time from 5 to 20 min in an oxidizing reactor, said        initial oxidizing catalytic system comprising at least three        compounds selected from the group consisting of cobalt compound,        manganese compound, zirconium compound and bromine compound;    -   b) conducting a second oxidation in situ at a temperature from        160 to 220° C. for a time from 30 to 60 min under addition of an        additional catalytic system, said additional catalytic system        comprising at least two compounds selected from the group        consisting of cobalt compound, manganese compound, zirconium        compound, and bromine compound; and    -   c) completing the oxidation of pseudocumene into trimellitic        acid at a temperature from 180 to 230° C. for a time from 5 to        20 min without the addition of catalysts into the reactor,    -   wherein a pressure is adjusted in the range from 100 to 450 psig        over the steps a), b) and c)

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a flow diagram schematically showing production of trimelliticacid from pseudocumene according to an embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

A more detailed description of a method of producing trimellitic acidwill be given, below.

As described above, the present invention pertains to a method ofproducing trimellitic acid through liquid-phase oxidation ofpseudocumene in an acetic acid solvent under a gas atmosphere containingmolecular oxygen using a combination of oxidizing catalytic ingredientsselected from the group consisting of cobalt, manganese, zirconium andbromine.

As such, the oxidation of pseudocumene into trimellitic acid isperformed through three steps. Preferably, the present process iscarried out in a discontinuous mode because the continuous mode is lessfavorable than the discontinuous mode in terms of the yield oftrimellitic acid and the efficiency of the overall process.

In accordance with the present invention, the oxidizing catalytic systemshould be employed for the first and the second oxidation steps. Thecatalytic ingredients group suitable for such oxidizing catalytic systemincludes cobalt compound, manganese compound, zirconium compound, andbromine compound, and are used in the combined form thereof. Theoxidizing catalytic system for the first oxidation step contains thecombinations of at least three catalytic ingredients, while theoxidizing catalytic system for the second oxidation step contains thecombinations of at least two catalytic ingredients.

The cobalt compound, the manganese compound, and the zirconium compoundare not restricted by specific examples if being dissolved in the aceticacid. Employable are organic acid salt such as acetate, propionate,naphthenate, and octenate, hydroxide, halide (e.g. chloride andbromide), and inorganic acid salt such as borate, nitrate and carbonate,and preferably acetate, phosphate, hydroxide and bromide, and morepreferably acetate.

Representatives of the bromine compound are bromine, hydrogen bromide,ammonium bromide, an alkali metal bromide such as sodium bromide,lithium bromide and potassium bromide, inorganic bromide such as cobaltbromide and manganese bromide, organic bromide such as ethanetetrabromide, acetyl bromide and benzyl bromide. Among them, sodiumbromide, hydrogen bromide, cobalt bromide, manganese bromide, andammonium bromide are preferably employed. Sodium bromide is morepreferable.

An amount of cobalt to pseudocumene is 0.1 to 0.4 wt % in the initialoxidizing catalytic system, and 0 to 0.2 wt % in the additionalcatalytic system.

An amount of manganese to pseudocumene is 0.01 to 0.1 wt % in theinitial oxidizing catalytic system, and 0.01 to 0.3 wt % in theadditional catalytic system.

An amount of zirconium to pseudocumene is 0 to 0.01 wt % in the initialoxidizing catalytic system, and 0 to 0.01 wt % in the additionalcatalytic system.

An amount of bromine to pseudocumene is 0.01 to 0.1 wt % in the initialoxidizing catalytic system, and 0.05 to 0.5 wt % in the additionalcatalytic system.

If an amount of each metal compound constituting is less than the aboveranges, trimellitic acid with desirable purity is not obtained due tothe reduction of the oxidation rate of pseudocumene. On the other hand,where the amount is unnecessarily large, the side reaction rate is soincreased as to promote the production of impurities, and lossesattributable to the complete oxidation of the acetic acid orpseudocumene are problematic.

Meanwhile, as the oxidation goes on, the cobalt, manganese, andzirconium in the oxidizing catalytic system form a complex salt with thetrimellitic acid, thus causing deactivation of the catalytic systems.This phenomenon makes it difficult to recycle the used catalysts.Furthermore, bromine substitutes a hydrogen atom of trimellitic acid toform bromo trimellitic acid. In addition, it is desirable to use assmall an amount of catalyst as possible so as to reduce catalyst cost.

In accordance with the present invention, the first, the second and thethird oxidation steps are carried out at the temperature from 120 to200° C., from 160 to 220° C., and from 180 to 230° C., respectively.Where each reaction temperature is lower than the above temperaturerange, an oxidation rate of pseudocumene is so reduced that the desiredreaction extent may not be obtained. On the other hand, where eachtemperature is higher than the above temperature range, the yield oftrimellitic acid is lowered due to the complete oxidation of acetic acidor pseudocumene.

The reaction pressure should be controlled in the range of 100 to 450psig over the three oxidation steps to maintain the acetic acid solventin a liquid phase within the above temperature range.

According to the present invention, the partial pressure of themolecular oxygen in the reacting system needs to be maintained in such amanner that the oxygen concentration in the gas discharged from theoxidizing reactor is in the range of about 2-8 volume %. For this, thereaction time for each of the three oxidation steps is controlled in therange of 5 to 20 min, 30 to 60 min, and 5 to 20 min. If the oxygenconcentration in the discharged gas is excessively high or low, the sideproducts are increased, thus reducing the yield of trimellitic acid. Inparticular, it should be noted that the oxygen concentration iscontrolled to less than 8 volume % in order to prevent the acetic acidsolvent from exploding.

A molar ratio of pseudocumene to the acetic acid solvent is 1:2 to 1:12,and preferably 1:4 to 1:10.

As described above, a portion of trimellitic acid produced at the earlystage of the oxidation reaction of pseudocumene causes deactivation ofthe catalytic system. Accordingly, the additional catalytic systemshould be added to the oxidizing reactor at the second oxidationreaction step so as to obtain trimellitic acid of high purity. Forsimilar reasons, it is preferred that the additional catalytic system isintroduced to the reactor as the mixture of the catalytic ingredients,rather than as a single catalytic ingredient.

Additionally, in the case of the third oxidation reaction, it ispreferred that the reaction temperature and pressure are increased andmaintained without the addition of the catalysts to the reactor tosuppress losses due to the complete oxidation of the solvent orpseudocumene. As a result, the oxidation of pseudocumene intotrimellitic acid is completed.

Typically, the produced trimellitic acid functions to form the complexsalt with the catalytic ingredients to reduce activity of the catalyst,which makes it difficult to recycle the used catalyst. However,according to the present invention, trimellitic acid of high purity maybe produced using a small amount of catalyst through three-stepoxidation reaction. In particular, the present inventors have noted thatwhen the purity of trimellitic acid is 96 wt % or more, major impuritiescontained in the product are neither dimethlybenzenecarboxylic acid normethylphthalic acid, but three isomers of phthalic acid. Considering theconcentration of impurities in the pseudocumene acting as a reactant, itis believed that the pseudocumene is converted into one of the isomersof xylene by drop of any one of three methyl groups therein duringoxidation of pseudocumene, and then oxidized to form three isomers ofphthalic acid. Accordingly, the present process is capable of improvingthe purity of trimellitic acid by suppressing the demethylation ofpseudocumene.

A better understanding of the present invention may be obtained in lightof the following examples that are set forth to illustrate, but are notto be construed to limit the present invention.

COMPARATIVE EXAMPLE 1

Pseudocumene was oxidized to produce trimellitic acid according to areaction system as shown in FIG. 1. In detail, pseudocumene as areactant and catalytic ingredients constituting a catalytic system weredissolved in acetic acid in a predetermined mixing ratio as described inTable 1, and the resulting solution was put into an oxidizing reactor.After air was removed from the oxidizing reactor using an inert nitrogengas, a temperature in the oxidizing reactor was increased to 140° C.,and the first oxidation reaction was conducted for 10 min while slowlyinjecting compressed air into the reactor under a constant pressure of250 psig. The temperature and pressure in the oxidizing reactor werethen increased to 205° C. and 365 psig, respectively, and the secondoxidation reaction was conducted for about 50 min while adding anadditional catalytic system to the reactor in a predetermined mixingratio as described in Table 1 to accomplish oxidation of pseudocumene.At this time, a total injection amount of compressed air was controlledso as to be 1.3 times as much as a theoretical amount of air required tooxidize pseudocumene to trimellitic acid. Purity of the obtainedtrimellitic acid is described in Table 1.

EXAMPLES 1-2 AND COMPARATIVE EXAMPLES 2-3

Pseudocumene as a reactant and catalytic ingredients constituting acatalytic system were dissolved in acetic acid in a predetermined mixingratio as described in Table 1, and the resulting solution was put intothe same oxidizing reactor as in Comparative Example 1. After air wasremoved from the oxidizing reactor using an inert nitrogen gas, atemperature in the oxidizing reactor was increased to 140° C., and thefirst oxidation reaction was conducted while slowly injecting compressedair into the reactor under constant pressure of 250 psig. After about 10min, the temperature in the oxidizing reactor was increased to thetemperature between 175 and 180° C., and the second oxidation reactionwas then started while adding an additional catalytic system in apredetermined mixing, ratio as described in Table 1 to the reactor.Thus, the second oxidation reaction was conducted at the temperaturebetween 190 and 195° C. for 40 min. The third oxidation reaction wasconducted at 205° C. under pressure of 365 psig for 10 min while slowlyreducing the amount of compressed air without the addition of anycatalysts to complete oxidation of the pseudocumene. At this time, atotal injection amount of compressed air was controlled to 1.3 times asmuch as the theoretical amount of air required to oxidize pseudocumeneto trimellitic acid. Purity of the obtained trimellitic acid isdescribed in Table 1. TABLE 1 unit Com. Ex. 1 Exam. 1 Exam. 2 Com. Ex. 2Com. Ex. 3 pseudocumene g 1000 1000 1400 1400 1400 cobalt/pseudocumenewt % 0.39 0.39 0.195 0.195 0.146 manganese/pseudocumene (1) wt % 0.0640.064 0.032 0.032 0.024 manganese/pseudocumene (2) wt % 0.256 0.2560.064 0.064 0.048 zirconium/pseudocumene (1) wt % 0.009 0.009 0.0045 00.0033 zirconium/pseudocumene (2) wt % 0.018 0.018 0.0045 0 0.0033bromine/pseudocumene (1) wt % 0.1 0.1 0.05 0.05 0.038bromine/pseudocumene (2) wt % 0.8 0.8 0.2 0.2 0.15 acetic acid g 50005000 3500 3500 3500 trimellitic acid (purity) % 92.2 97.52 97.83 80.8284.91(1): first oxidation reaction(2): second oxidation reaction

From the results of Table 1, it can be seen that purity of trimelliticacid produced through the two-step oxidation reactions according toComparative Example 1 is very poor in comparison with the trimelliticacid produced through three-step oxidation reactions according toExample 1, even though employing the same catalytic system. Accordingly,it is confirmed that the third oxidation reaction of pseudocumene has agreat influence on the overall efficiency, contrary to the teaching ofthe prior arts in which trimellitic acid is efficiently produced throughtwo-step oxidation reaction. In the case of Example 2, purity oftrimellitic acid is not reduced even though an amount of the catalyst topseudocumene is half of that in Example 1.

In Comparative Example 2, the oxidation of pseudocumene is performedusing the initial and additional catalytic systems not containingzirconium. As a result, the purity of the resulting trimellitic acid isgreatly reduced. Accordingly, it can be seen that zirconium is requiredto desirably produce trimellitic acid. Additionally, in the case ofComparative Example 3, when an amount of each component constituting thecatalytic systems is 75% of that of Example 2, the purity of trimelliticacid is greatly reduced due to an insufficient amount of catalyticingredients.

Consequently, catalytic and reaction conditions of Example 2 yieldingtrimellitic acid with a purity of 97.82% are most preferable to oxidizepseudocumene into trimellitic acid.

COMPARATIVE EXAMPLES 4 AND 5

Trimellitic acid was produced by oxidizing pseudocumene using a reactionsystem as shown in FIG. 1 in a continuous mode.

Oxidation reaction conditions of pseudocumene and the results ofoxidation reaction of pseudocumene are given in Table 2. Pseudocumenewas oxidized into trimellitic acid using the oxidizing catalytic systemsconsisting of cobalt, manganese, and bromine through two oxidationreactions having different conditions of temperature and pressure. Amongplural oxidation reaction experiments, reaction conditions and purity oftrimellitic acid with the highest purity are described in Table 2. TABLE2 Com. Ex. 4 Com. Ex. 5 1^(st) 2^(nd) 1^(st) 2^(nd) unit reactionreaction reaction reaction pseudocumene g 904.50 2713.5cobalt/pseudocumene wt % 0.86 0.21 0.86 0.21 manganese/ wt % 0.44 0.110.44 0.11 pseudocumene bromine/ wt % 1.66 0.40 1.66 0.40 pseudocumeneacetic acid g 17185.5 51556.5 pressure psig 85 280 85 280 temperature °C. 141 201 142 208 reaction time min 60 60 60 90 trimellitic acid %53.43 82.20 (purity)

From the results of Table 2, it can be seen that the continuous processdoes not desirably accomplish the oxidation reaction of pseudocumene totrimellitic acid even though having various advantages.

INDUSTRIAL APPLICABILITY

As described above, the present invention is advantageous in that ademethylation of pseudocumene is greatly suppressed unlike theconventional method of producing trimellitic acid through the two-stepoxidation reactions, thereby improving purity of the trimellitic acid.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method for producing trimellitic acid through liquid-phaseoxidation of pseudocumene in an acetic acid solvent under a gasatmosphere containing molecular oxygen in the presence of a catalyticsystem containing at least one selected from the group consisting ofcobalt, manganese, zirconium and bromine, said oxidation of pseudocumenecomprising: a) conducting a first oxidation in the presence of aninitial oxidizing catalytic system at a temperature from 120 to 200° C.for a time from 5 to 20 min in an oxidizing reactor, said initialoxidizing catalytic system comprising at least three compounds selectedfrom the group consisting of cobalt compound, manganese compound,zirconium compound and bromine compound; b) conducting a secondoxidation in situ at a temperature from 160 to 220° C. for a time from30 to 60 min under addition of an additional catalytic system, saidadditional catalytic system comprising at least two compounds selectedfrom the group consisting of cobalt compound, manganese compound,zirconium compound, and bromine compound; and c) completing theoxidation of pseudocumene into trimellitic acid at a temperature from180 to 230° C. for a time from 5 to 20 min without the addition ofcatalysts into the reactor, wherein a pressure is adjusted in the rangefrom 100 to 450 psig over the steps a), b) and c).
 2. The method as setforth in claim 1, wherein an amount of cobalt is 0.1 to 0.4 wt %, anamount of manganese is 0.01 to 0.1 wt %, an amount of zirconium is 0 to0.01 wt %, and an amount of bromine is 0.01 to 0.1 wt % based on aweight of pseudocumene in the initial oxidizing catalytic system.
 3. Themethod as set forth in claim 1, wherein an amount of cobalt is 0 to 0.2wt %, an amount of manganese is 0.01 to 0.3 wt %, an amount of zirconiumis 0 to 0.01 wt %, and an amount of bromine is 0.05 to 0.5 wt % based ona weight of pseudocumene in the additional catalytic system.
 4. Themethod as set forth in claims 1, wherein the oxygen concentration in thegas discharged from the oxidizing reactor is maintained in the range of2 to 8 volume %.
 5. The method as set forth in claim 1, wherein a molarratio of the pseudocumene to the acetic acid solvent is 1:2 to 1:12. 6.The method as set forth in claim 1, wherein the step c) is performedunder the condition of higher temperature and pressure than the step b).7. The method as set in claim 1, wherein the method is carried out in adiscontinuous mode.